Antenna Circuit

ABSTRACT

In an antenna circuit including an antenna  1  and an antenna  2  that is connected in series with the antenna  1  and includes inductance, a variable capacitor C v  and a variable resistor R v  connected in parallel with the antenna  2  are provided. This enables controlling of an actual amplitude ratio r and a phase difference θ between currents I 1  and I 2  flowing through the two antennas  1  and  2  into desired values. Flows of the currents I 1  and I 2  with the phase difference θ through the antennas  1  and  2  enable forming of a favorable communication area. In addition, setting of the actual amplitude ratio r between the currents I 1  and I 2  flowing through the antennas  1  and  2  to a value other than 1 enables forming of an asymmetric communication area.

TECHNICAL FIELD

The present invention relates to a wireless communication technologythat enables desired setting of a boundary of a communication area.

BACKGROUND ART

The needs of a wireless communication system that limits a communicationarea as desired have been increased recently. A system using a magneticfield has been in practical use as such a wireless communication system.A loop antenna and a bar antenna have been used for generating themagnetic field. A disclosed effective way in generating a magnetic fielddesired by a designer is to use multiple antennas and appropriatelycontrol phases of currents flowing through those antennas (Patentdocument 1).

PRIOR ART DOCUMENT Patent Document

-   Patent document 1: Japanese Patent No. 6059833-   Patent document 2: Japanese Patent No. 6069548

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In general, the technology using multiple antennas usually employs aconfiguration in which the multiple antennas are connected in series(Patent document 2). Such a series connection configuration requiresonly one signal source, which is an advantage. However, in the simpleseries connection configuration disclosed in Patent document 2,amplitude and phases of currents flowing through the antennas naturallytake the same values in all the antennas.

Thus, there is a problem that multiple signal sources are needed inorder to control the amplitude and phases of the currents flowingthrough the antennas according to the proposition of Patent document 1.

The present invention is made in light of the above-described problem,and has an object to control the amplitude and phases of the currentsflowing through the multiple antennas connected in series into desiredvalues.

Means for Solving the Problem

An antenna circuit according to an aspect of the present inventionincludes: a first antenna; a second antenna that is connected in serieswith the first antenna and includes inductance; an adjustment capacitorthat is connected in parallel with the second antenna; and an adjustmentresistor that is connected in parallel with the second antenna.

Effect of the Invention

According to the present invention, it is possible to control amplitudeand phases of currents flowing through multiple antennas connected inseries into desired values.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram that illustrates a configuration example of anantenna circuit of an embodiment.

FIG. 2 is a diagram that illustrates an equivalent circuit of theantenna circuit in FIG. 1.

FIG. 3 is a diagram that illustrates current waveforms in a simulationon the equivalent circuit in FIG. 2.

FIG. 4 is a diagram that illustrates an arrangement example of antennas.

FIG. 5 is a diagram that illustrates magnetic field distributionobtained when currents with an amplitude ratio of 1 and no phasedifference are applied to the antennas in FIG. 4.

FIG. 6 is a diagram that illustrates magnetic field distributionobtained when the currents illustrated in FIG. 3 are applied to theantennas in FIG. 4.

FIG. 7 is a diagram that illustrates other current waveforms in asimulation on the equivalent circuit in FIG. 2.

FIG. 8 is a diagram that illustrates magnetic field distributionobtained when the currents illustrated in FIG. 7 are applied to theantennas in FIG. 4.

FIG. 9 is a diagram that illustrates a configuration example of anotherantenna circuit of the embodiment.

FIG. 10 is a diagram that illustrates an equivalent circuit of theantenna circuit in FIG. 9.

FIG. 11 is a diagram that illustrates current waveforms in a simulationon the equivalent circuit in FIG. 10.

FIG. 12 is a diagram that illustrates other current waveforms in asimulation on the equivalent circuit in FIG. 10.

MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention is described below with referenceto the drawings.

FIG. 1 is a diagram that illustrates a configuration example of anantenna circuit of this embodiment.

The antenna circuit illustrated in FIG. 1 includes multiple antennas 1and 2 connected in series, a signal source 3, and a variable capacitorC_(v) and a variable resistor R_(v) connected in parallel with theantenna 2.

In the antenna circuit illustrated in FIG. 1, the antennas 1 and 2 areconnected such that currents therethrough flow in different directionsfrom each other. However, the antennas 1 and 2 may be connected suchthat the currents therethrough flow in the same direction. Complexamplitude of a current I₂ flowing through the antenna 2 varies accordingto a capacitance value of the variable capacitor C_(v) and a resistancevalue of the variable resistor R_(v) and differs from complex amplitudeof a current I₁ flowing through the antenna 1. Appropriate setting ofthe capacitance value of the variable capacitor C_(v) and the resistancevalue of the variable resistor R_(v) enables controlling of an actualamplitude ratio (r=|I₁/I₂|) and a phase difference (θ=arg (I₁/I₂))between the currents flowing through the two antennas 1 and 2 intodesired values. Unless otherwise stated, the symbols I₁ and I₂ denotingthe currents represent the complex amplitude.

FIG. 2 illustrates an equivalent circuit of the antenna circuit inFIG. 1. Z₁ represents impedance of the antenna 1. L₂ representsinductance of the antenna 2.

The actual amplitude ratio r and the phase difference θ between thecurrents are indicated by the following equations (1) and (2).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack & \; \\{r = {{{1 - {\omega^{2}L_{2}C_{v}}}} \cdot \sqrt{1 + \left( \frac{\omega \; {L_{2}/R_{v}}}{1 - {\omega^{2}L_{2}C_{v}}} \right)^{2}}}} & (1) \\\left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack & \; \\{\theta = {\arctan \left( \frac{\omega \; {L_{2}/R_{v}}}{1 - {\omega^{2}L_{2}C_{v}}} \right)}} & (2)\end{matrix}$

In the equations, ω is an angular frequency of a signal generated by thesignal source 3.

Both the equations (1) and (2) are independent of the impedance Z₁ ofthe antenna 1. Thus, an arbitrary antenna can be used as the antenna 1.The antenna 2 may be at least an antenna having inductance such as aloop antenna and a bar antenna. In the example of FIG. 1, the antennas 1and 2 have the same shape; however, they may have different shapes.

Now, a value C_(v) ^(opt) of the variable capacitor C_(v) and a valueR_(v) ^(opt) of the variable resistor R_(v) required to achieve desiredactual amplitude ratio r₀ and phase difference θ₀ between the currentsare described.

The actual amplitude ratio r₀ and phase difference θ₀ are substitutedinto the equations (1) and (2) to solve for C_(v) ^(opt) and R_(v)^(opt), and thus the following equations (3) and (4) can be obtained.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack & \; \\{C_{v}^{opt} = \frac{1 - {r_{0}\cos \; \theta_{0}}}{\omega^{2}L_{2}}} & (3) \\\left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack & \; \\{R_{v}^{opt} = \frac{\omega \; L_{2}}{r_{0}\sin \; \theta_{0}}} & (4)\end{matrix}$

The desired actual amplitude ratio r₀ and phase difference θ₀ can beachieved by setting the value of the variable capacitor C_(v) of theantenna circuit in FIG. 1 to a value obtained from the equation (3) andsetting the value of the variable resistor R_(v) of the antenna circuitin FIG. 1 to a value obtained from the equation (4).

For example, it is assumed that the desired actual amplitude ratio r₀and phase difference θ₀ are values of the following equation (5).

[Math. 5]

r ₀=1,θ₀=11 deg  (5)

In addition, it is assumed that a signal frequency f of the signalsource 3 and the inductance L₂ of the antenna 2 are values of thefollowing equation (6).

[Math. 6]

f=1 MHz,L ₂=10 μH  (6)

The equations (5) and (6) are substituted into the equations (3) and (4)to calculate appropriate parameters, and thus the following values C_(v)^(opt) and R_(v) ^(opt) can be obtained.

[Math. 7]

C _(v) ^(opt)=46.5389 pF,R _(v) ^(opt)=329.292Ω  (7)

FIG. 3 illustrates waveforms of the currents I₁ and I₂ in a simulationon the equivalent circuit in FIG. 2 using the parameters of theequations (6) and (7). An amplitude of voltage applied from the signalsource 3 is 1V.

According to FIG. 3, it can be seen that the amplitude ratio between thecurrents I₁ and I₂ is 1 and a time difference therebetween is 31 ns(comparable to 11 deg). In the antenna circuit of this embodiment,employment of the values C_(v) ^(opt) and R_(v) ^(opt) calculated fromthe equations (3) and (4) enables controlling of the amplitude ratio andphase difference between the currents I₁ and I₂ flowing through theantennas 1 and 2 as desired.

Now, effects of the current phase difference on magnetic fielddistribution are described.

As illustrated in FIG. 4, it is assumed that the antennas 1 and 2 havingthe same shape are disposed in an x-y plane. It is assumed thatdirections of the currents I₁ and I₂ flowing through the antennas 1 and2 are different from each other.

FIG. 5 is a diagram that illustrates magnetic field distributionobtained when currents with the amplitude ratio of 1 and no phasedifference are applied to the antennas 1 and 2 illustrated in FIG. 4.FIG. 6 is a diagram that illustrates magnetic field distributionobtained when the currents I₁ and I₂ illustrated in FIG. 3 are appliedto the antennas 1 and 2 illustrated in FIG. 4. Each contour in FIGS. 5and 6 indicate a line plotting points where a magnetic field strengthhas a certain value at a y-coordinate of 0, 50, 60, 70, or 80 cm.

In FIG. 5, the contour is divided when a value of the y-coordinate islarge, and this is a problem in forming a communication area. On theother hand, in FIG. 6, the contour is not divided even when the value ofthe y-coordinate is large, and this enables forming of a favorablecommunication area.

Next, an example of forming asymmetric magnetic field distribution isdescribed.

As illustrated in FIG. 5, the magnetic field distribution obtained bythe conventional antenna circuit configuration is symmetric about a y-zplane. In addition, as illustrated in FIG. 6, changing of only phases ofthe currents I₁ and I₂ flowing through the antennas 1 and 2 still makesthe magnetic field distribution symmetric about the y-z plane. However,asymmetric magnetic field distribution may be desired in someapplication.

In the antenna circuit of this embodiment, setting of both the amplitudeand the phases of the currents I₁ and I₂ flowing through the antennas 1and 2 to different values enables creating of magnetic fielddistribution asymmetric about the y-z plane.

For example, it is assumed that the currents I₁ and I₂ satisfying thefollowing equation (8) flow through the antennas 1 and 2. Note that, itis assumed that the signal frequency f of the signal source 3 and theinductance L₂ of the antenna 2 are the values of the equation (6).

[Math. 8]

r ₀=1/1.3,θ₀=10 deg  (8)

The equations (6) and (8) are substituted into the equations (3) and (4)to calculate appropriate parameters, and thus the following values C_(v)^(opt) and R_(v) ^(opt) can be obtained.

[Math. 9]

C _(v) ^(opt)=614.147 pF,R _(v) ^(opt)=470.384Ω  (9)

FIG. 7 illustrates waveforms of the currents I₁ and I₂ in a simulationon the equivalent circuit in FIG. 2 using the parameters of theequations (6) and (9). An amplitude of voltage applied from the signalsource 3 is 1V.

According to FIG. 7, it can be seen that the amplitude ratio of thecurrents I₁ and I₂ is 1/1.3 and a time difference therebetween is 28 ns(comparable to 10 deg).

FIG. 8 is a diagram that illustrates magnetic field distributionobtained when the currents I₁ and I₂ illustrated in FIG. 7 are appliedto the antennas 1 and 2 illustrated in FIG. 4. As illustrated in FIG. 8,it can be seen that magnetic field distribution asymmetric about the y-zplane is formed.

Next, another antenna circuit of this embodiment is described.

FIG. 9 is a diagram that illustrates a configuration example of theother antenna circuit of this embodiment.

The antenna circuit illustrated in FIG. 9 is different from the antennacircuit in FIG. 1 in that a resistor R₁ and a capacitor C₁ as well as aresistor R₂ and a capacitor C₂ are connected in series with the antenna1 and the antenna 2 respectively. The variable capacitor C_(v) and thevariable resistor R_(v) are connected in parallel with the componentsincluding the antenna 2, the resistor R₂, and the capacitor C₂.

When a loop antenna is used as each of the antennas 1 and 2, a capacitoris connected in series with the antenna in order to reduce the impedancedue to the inductance of the loop antenna. In some cases, a resistor isintentionally connected in order to decrease a quality factor of theantenna.

FIG. 10 illustrates an equivalent circuit of the antenna circuit in FIG.9. For the equivalent circuit in FIG. 10, the capacitance value C_(v)^(opt) of the variable capacitor C_(v) and the resistance value R_(v)^(opt) of the variable resistor R_(v) for achieving the desired actualamplitude ratio r₀ and phase difference θ₀ between the currents can beobtained from the following equations (10) and (11).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 10} \right\rbrack & \; \\{C_{v}^{opt} = {C_{2} \cdot \frac{{\omega \; C_{2}R_{2}r_{0}\sin \; \theta_{0}} + {\left( {{\omega^{2}L_{2}C_{2}} - 1} \right)\left( {1 - {r_{0}\cos \; \theta_{0}}} \right)}}{1 + \left( {\omega \; C_{2}R_{2}} \right)^{2} + {\omega^{2}L_{2}{C_{2}\left( {{\omega^{2}L_{2}C_{2}} - 2} \right)}}}}} & (10) \\\left\lbrack {{Math}.\mspace{14mu} 11} \right\rbrack & \; \\{R_{v}^{opt} = {\frac{1}{\omega \; C_{2}} \cdot \frac{1 + \left( {\omega \; C_{2}R_{2}} \right)^{2} + {\omega^{2}L_{2}{C_{2}\left( {{\omega^{2}L_{2}C_{2}} - 2} \right)}}}{{{\omega \; C_{2}{R_{2}\left( {{r_{0}\cos \; \theta_{0}} - 1} \right)}} + {r_{0}\sin \; {\theta_{0}\left( {{\omega^{2}L_{2}C_{2}} - 1} \right)}}}\;}}} & (11)\end{matrix}$

For example, it is assumed that the desired actual amplitude ratio r₀and phase difference θ₀ are values of the following equation (12).

[Math. 12]

r ₀=1,θ₀=11 deg  (12)

In addition, it is assumed that the signal frequency f of the signalsource 3 and the inductance L₂, the capacitors C₁ and C₂, and theresistors R₁ and R₂ of the antenna 2 have values of the followingequation (13).

[Math. 13]

f=1 MHz,L ₁ =L ₂=10 μH,C ₁ =C ₂=2800 pF,R ₁ =R ₂=10Ω  (13)

The equations (12) and (13) are substituted into the equations (10) and(11) to calculate appropriate parameters, and thus the following valuesC_(v) ^(opt) and R_(v) ^(opt) can be obtained.

[Math. 14]

C _(v) ^(opt)=2363.68 pF,R _(v) ^(opt)=141.645Ω  (14)

FIG. 11 illustrates waveforms of the currents I₁ and I₂ in a simulationon the equivalent circuit in FIG. 10 using the parameters of theequations (13) and (14). An amplitude of voltage applied from the signalsource 3 is 1V.

According to FIG. 11, it can be seen that the amplitude ratio of thecurrents I₁ and I₂ is 1 and a time difference therebetween is 31 ns(comparable to 11 deg). In addition, as an effect of connecting thecapacitors C₁ and C₂ in series with the antennas 1 and 2 respectively,the amplitude of the currents I₁ and I₂ is greater than that in FIG. 3.

Next, an example where the amplitude ratio r₀ is a value other than 1(r₀≠1) in the antenna circuit in FIG. 9 is described.

For example, it is assumed that the currents I₁ and I₂ satisfying thefollowing equation (15) flow through the antennas 1 and 2.

[Math. 15]

r ₀=1.3,θ₀=11 deg  (15)

Note that, it is assumed that the signal frequency f of the signalsource 3 and the inductance L₂, the capacitors C₁ and C₂, and theresistors R₁ and R₂ of the antenna 2 have the values of the equation(13).

The equations (12) and (13) are substituted into the equations (10) and(11) to calculate appropriate parameters, and thus the following valuesC_(v) ^(opt) and R_(v) ^(opt) can be obtained.

[Math. 16]

C _(v) ^(opt)=967.844 pF,R _(v) ^(opt)=31.9953Ω  (16)

FIG. 12 illustrates waveforms of the currents I₁ and I₂ in a simulationon the equivalent circuit in FIG. 10 using the parameters of theequations (13) and (16). An amplitude of voltage applied from the signalsource 3 is 1V.

According to FIG. 12, it can be seen that the amplitude ratio of thecurrents I₁ and I₂ is 1.3 and a time difference therebetween is 31 ns(comparable to 11 deg). In addition, as an effect of connecting thecapacitors C₁ and C₂ in series with the antennas 1 and 2 respectively,the amplitude of the currents I₁ and I₂ is greater than that in FIG. 7.

As described above, in the antenna circuit in FIG. 9, employment of thevalues C_(v) ^(opt) and R_(v) ^(opt) calculated from the equations (10)and (11) enables controlling of the amplitude ratio and phase differencebetween the currents I₁ and I₂ flowing through the antennas 1 and 2 asdesired and also allows large currents to flow through the antennas 1and 2.

As described above, according to this embodiment, in the antenna circuitincluding the antenna 1 and the antenna 2 that is connected in serieswith the antenna 1 and has inductance, the variable capacitor C_(v) andthe variable resistor R_(v) connected in parallel with the antenna 2 areprovided, and this enables controlling of the actual amplitude ratio rand the phase difference θ between the currents I₁ and I₂ flowingthrough the two antennas 1 and 2 into desired values. Flows of thecurrents I₁ and I₂ with the phase difference θ through the antennas 1and 2 enables forming of a favorable communication area. In addition,setting of the actual amplitude ratio r between the currents I₁ and I₂flowing through the antennas 1 and 2 to a value other than 1 enablesforming of an asymmetric communication area.

REFERENCE SIGNS LIST

-   -   1, 2 antenna    -   3 signal source    -   C_(v) variable capacitor    -   R_(v) variable resistor    -   C₁, C₂ capacitor    -   R₁, R₂ resistor

1. An antenna circuit, comprising: a first antenna; a second antennathat is connected in series with the first antenna and includesinductance; an adjustment capacitor that is connected in parallel withthe second antenna; and an adjustment resistor that is connected inparallel with the second antenna, wherein desired amplitude ratio r₀ andphase difference θ₀ are obtained under conditions where a capacitancevalue of the adjustment capacitor is expressed by:$\frac{1 - {r_{0}\cos \; \theta_{0}}}{\omega^{2}L_{2}}$ and aresistance value of the adjustment resistor is expressed by:$\frac{\omega \; L_{2}}{r_{0}\sin \; \theta_{0}}$ where r₀ is anamplitude ratio between currents flowing through the first antenna andthe second antenna, θ₀ is a phase difference between the currentsflowing through the first antenna and the second antenna, ω is anangular frequency of the currents flowing through the first antenna andthe second antenna, and L₂ is the inductance of the second antenna. 2.(canceled)
 3. (canceled)
 4. An antenna circuit comprising: a firstantenna; a second antenna that is connected in series with the firstantenna and includes inductance: a capacitor connected in series withthe second antenna; a resistor connected in series with the secondantenna; an adjustment capacitor connected in parallel with a circuitincluding the second antenna, the capacitor, and the resistor, and anadjustment resistor connected in parallel with the circuit including thesecond antenna, the capacitor, and the resistor.
 5. The antenna circuitaccording to claim 4, wherein desired amplitude ratio r₀ and phasedifference θ₀ are obtained under conditions where a capacitance value ofthe adjustment capacitor is expressed by$C_{2} \cdot \frac{{\omega \; C_{2}R_{2}r_{0}\sin \; \theta_{0}} + {\left( {{\omega^{2}L_{2}C_{2}} - 1} \right)\left( {1 - {r_{0}\cos \; \theta_{0}}} \right)}}{1 + \left( {\omega \; C_{2}R_{2}} \right)^{2} + {\omega^{2}L_{2}{C_{2}\left( {{\omega^{2}L_{2}C_{2}} - 2} \right)}}}$and a resistance value of the adjustment resistor is expressed by$\frac{1}{\omega \; C_{2}} \cdot \frac{1 + \left( {\omega \; C_{2}R_{2}} \right)^{2} + {\omega^{2}L_{2}{C_{2}\left( {{\omega^{2}L_{2}C_{2}} - 2} \right)}}}{{{\omega \; C_{2}{R_{2}\left( {{r_{0}\cos \; \theta_{0}} - 1} \right)}} + {r_{0}\sin \; {\theta_{0}\left( {{\omega^{2}L_{2}C_{2}} - 1} \right)}}}\;}$to obtain desired amplitude ratio r₀ and phase difference θ₀ where r₀ isan amplitude ratio between currents flowing through the first antennaand the second antenna, θ₀ is a phase difference between the currentsflowing through the first antenna and the second antenna, ω is anangular frequency of the currents flowing through the first antenna andthe second antenna, L₂ is the inductance of the second antenna, C₂ isthe capacitance value of the adjustment capacitor, and R₂ is theresistance value of the adjustment resistor.
 6. The antenna circuitaccording to claim 1, wherein the amplitude ratio r₀ is
 1. 7. Theantenna circuit according to claim 1, wherein the first antenna and thesecond antenna are in a same shape.
 8. The antenna circuit according toclaim 5, wherein the amplitude ratio r₀ is
 1. 9. The antenna circuitaccording to claim 4, wherein the first antenna and the second antennaare in a same shape.